1. Field of the Invention
The present invention relates to Broadband/Ultra-wideband (UWB) antenna design.
2. Description of the Related Art
Ultra-Wideband (UWB) communication has been the subject of intense research over the last few years. The essence of UWB systems is the ability to transmit and receive UWB pulses, which occupy a bandwidth over several octaves. A UWB system needs an antenna that maintains good phase and amplitude linearity over a wide bandwidth.
Broadband antennas have been studied in the past for short pulse applications. Basically, there are two ways to achieve broadband functionality in an antenna. One is to broaden the bandwidth of currently available antennas, i.e., using one radiation element to cover the entire UWB bandwidth. The other approach is to use an antenna array for UWB applications. The antenna array is made of several radiation elements, with each of which covering a relatively narrow bandwidth, with their sum of bandwidths complying with the UWB requirements.
FIG. 1 shows a conventional 2-element Log-periodic Dipole Array (LPDA) 100 in schematic form. In general, an LPDA is a broadband, multi-element, unidirectional, narrow-beam antenna with impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequencies. The individual radiation elements in LPDA are dipole antennas. In a LPDA, there are several radiation elements or dipoles (for example, radiation element 1 (102) and radiation element 2 (104)), each of which covers a narrow bandwidth, and the LPDA 100 uses a single transmission line 108 to connect all the radiation elements (e.g., the two elements 102, 104) in order to achieve broader bandwidth.
Assume that element 1 (102) has a resonant frequency f1, and that element 2 (104) has a resonant frequency f2. If signals 106 with frequencies f1 and f2 are fed into the LPDA 100 at the same time, signals with frequency f1 will be radiated into free space by element 1 (102) while signals with frequency f2 will move along the transmission line 108 further since frequency f2 is not the resonant frequency of element 1 (102). Signals with frequency f2 will experience some additional delay caused by the transmission line 108 until it is radiated into the free space by element 2 (104). Obviously, such a radiation mechanism would introduce a non-constant group delay, i.e., non-linear phase characteristics.
Such non-linear phase characteristic will be even worse if a pair of LPDAs is used for UWB signal transmission and reception. FIG. 2 shows an example of using the LPDAs 100, 130 as the transmitter and receiver, respectively. Note that the elements 122, 124 in the LPDA 130 on the receiver side are arranged in orientation to the transmission line 128 identically to the way the elements 102, 104 in the LPDA 100 on the transmitter side are arranged in orientation to the transmission line 108. Because of the non-linear phase characteristics, signals with frequency f1 are radiated first and signals with frequency f2 are radiated later with a delay caused by the transmission line 108. As a result, the signal with frequency f1 arrives at the receiver LPDA 130 earlier than the signals with frequency f2. In addition, signals with frequency f2 travel further along the transmission line 128 until it reaches its signal output 120, adding an extra delay between the signals with frequency f1 and the signals with frequency f2. Therefore, the original signals cannot be recovered.
FIGS. 3 and 4 show another conventional antenna array 300, referred to as Independently Center-fed Dipole Array (ICDA), for ultra-wideband applications, in schematic form. The ICDA also uses several narrowband radiation elements (e.g., two radiation elements 302, 304) in order to cover a broad bandwidth. However, the feed network 308 in the ICDA is different from that in LPDAs. Instead of having all the dipole elements serially connected by a transmission line, each element 302, 304 in the ICDA is fed independently through its own transmission line 320, 322, and all the transmission lines 320, 322 are connected at a splitting point 314 to the common transmission line 308 coupled to the input signal source 306. In other words, a broadband signal would travel on transmission line 308, be split up at the splitting point 314, and then fed into all the dipole elements 302, 304 via separate transmission lines 320, 322. By using the same transmission line 308 for both elements 302, 304 and then splitting up to separate transmission lines 320, 322 with equal length at the splitting point 314, all frequency components of the signal will be simultaneously fed into and radiated out by the corresponding active elements 302, 304.
Although the ICDA has linear phase characteristics, it also has low radiation efficiency. FIG. 4 shows an ICDA with N radiation elements. Referring to FIG. 4, the input signal 310 would travel on transmission line 308, and then be split up at junction 314 to N waves on separate transmission lines 320, 322, and propagate to each port corresponding to each radiation element (302, 304 . . . ). Thus, each radiation element would receive only a small portion of the original incident wave 310. For example, the incident wave 312 that is transmitted to element 1 (302) is only a small portion of the original incident wave 310. Thus, radiation efficiency is low in ICDAs.